Various pyrazolone derivatives have been known as magenta color image-forming couplers (referred to hereinafter simply as "magenta couplers"). However, these pyrazolone derivatives generally have low color forming efficiency (ratio of conversion of the coupler into a dye) when contained in photographic light-sensitive materials, and so-called 4-equivalent couplers, in which the coupling active position is not substituted, usually form only about 1/2 mol of dye per mol of the coupler.
To improve color forming efficiency, so-called 2-equivalent magenta couplers have been known, in which a substituent is introduced into the coupling active position of a pyrazolone type magenta coupler, and the substituent splits off in the color development step. Examples of such couplers are disclosed, for instance, in U.S. Pat. Nos. 3,311,476, 3,419,391, 3,617,291, 3,926,631, etc. Further, magenta couplers in which a substituent is connected to the coupling active position through a sulfur atom are described in U.S. Pat. No. 3,214,437 (a thiocyano group), U.S. Pat. No. 4,032,346 (an acylthio group or a thioacylthio group), U.S. Pat. Nos. 3,227,554 and 3,701,783 and Japanese Patent Publication No. 34044/78 (an arylthio group or a heterocyclic thio group), and West German Patent Application (OLS) No. 2,944,601 (an alkylthio group).
It has been found that when couplers having an arylthio group at the coupling active position among the magenta couplers described in U.S. Pat. Nos. 3,227,554 and 3,701,783 are used in a color photographic light-sensitive material and color images are formed, the light fastness of the color images does not completely satisfy the desired ever-advancing improvement in the properties of the color photographic light-sensitive materials.
Also, it has been found that when magenta couplers which release an arylthio group as described in Japanese Patent Publication No. 34044/78 are used in a color photographic light-sensitive material and color images are formed, the light fastness of the color images is insufficient.
Magenta couplers which release an arylthio group as described in Japanese Patent Application (OPI) No. 35858/82 (corresponding to U.S. Pat. No. 4,351,897) (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") overcome these known defects described above.
However, these known magenta couplers having an arylthio group as a splitting-off group as described above are disadvantageous in that the color forming property thereof is decreased when photographic light-sensitive materials containing such magenta couplers are processed in a color developing solution containing a salt of alkaline earth metal, such as calcium or magnesium. This can be a fatal defect where the processing solution is prepared using water containing a large amount of a salt of alkaline earth metal, that is, hard water. In fact, it is rarely the case that soft water can be used at color photographic processing laboratories in most parts of the world, and hard water is thus employed as the base water for processing solutions in most cases. For this reason color photographic light-sensitive materials containing these heretofore known magenta couplers having an arylthio group as a splitting-off group can be subjected to development processing only at specific color laboratories wherein hard water is not used, even though they do have several desirable properties.